In the turbine section of gas turbine engine, as well as in other parts, and in other turbomachinery, very close clearances are obtained between the spinning blades of a rotor and the circumscribing structure of the engine case. Occasionally, the tips will come into contact with the circumscribing parts, ordinarily called the seal segments, or simply, seals. To preserve the close clearances necessary for efficient engine operation, experience has shown that this must occur without significant wear of the blade tips. Thus, there has been developed a technology whereby an abradable material is applied to the interior of the case and the tips of the blades are made comparatively wear resistant.
In the pursuit of higher operating temperatures, the friable metals which originally comprised the seals have been replaced by ceramic materials. Even though such material are friable compared to monolithic ceramics, they can cause undue wear on turbine blades. Therefore, it has become the practice to apply to the tips of such blades ceramic particulate containing materials, such as the silicon carbide and superalloy metal matrix material described in commonly owned U.S. Pat. No. 4,243,913 of Johnson et al. The Johnson material is made by hot pressing and sintering a mixture of metal and ceramic powders, and joining the resultant material to the tip of a blade by welding, using transient liquid phase bonding or brazing.
The separately formed abrasive has limitations. Among them are that the forming of the separate piece and ensuring a good bonding surface can be costly; and, that when there is more than 15 volume percent ceramic in the material there is a propensity for cracking. There is also some tendency for failure at the point where the abrasive is bonded.
Others have also made abrasives for protecting the tips of turbine blades. For example, Zelahy et al. in U.S. Pat. No. 4,148,494 describe an electrodeposited combination. Stalker et al. in U.S. Pat. Nos. 4,227,703, 4,169,020 and 4,232,995 describe the use of a composite material structure at the tip in combination with an electrodeposited abrasive surface layer.
Commonly owned patent applications Ser. Nos. 624,446 and 624,421 of Novak et al. disclose plasma sprayed tip abrasives where the ceramic particulate is only one particle thick. The design of turbine blade tips has also been the subject of considerable work, aimed at improving the performance of tips. For example, see the aforementioned Stalker et al. patents and U.S. Pat. No. 4,390,320 to Eiswerth.
Because of the presence of ceramic material and the choice of matrices principally for their ability to hold the ceramic material, the abrasive material as a whole tends to have a different bulk thermal expansion from the superalloy substrate of the turbine blade. Since the use of turbine blades inherently subjects them to thermal cycling, significant cyclic strains are created where the abrasive material and substrate join, and these strains can lead to an undesired failure mode. Similarly, the abrasive material, being inhomogeneous, tends itself to be more prone to internal thermal strains and failure in regions of high temperature differential. For example, after a long period of use, cracks may be caused at the corner edge of the abrasive material at its outer or free surface.
Thus, there is a continuing need for improvements in the field, to obtain good durability with low manufacturing costs.